The function f(x) has the property that f (x+y) = f(x)+f(y)+xy, for all positive integers x and y. If f(1) = 4, then what is the value of f(8)?
f(1) = 4
f(1+1) = f(1) + f(1) + 1*1
f(2) = 9
f(2+2) = f(2) + f(2) + 2*2
f(4) = 22
f(4+4) = f(4) + f(4) + 4*4
f(8) = 60
=^._.^=
